Sneaky

I upgraded my contract the end of February to a free me 2gb. My previous contract had a R1 limit on and i had no issues. Its been 2months and i have been paying my line as normal with the R1 limit on(the contract was more then previous as i went from s9+ to s20+) I made sure when i upgraded two months ago to ask for my limit to remain at R1 and was told it's already set. I have now seen my current account figure is R163.80! How and why was this now set to R1500??? Why 2 months down the line it's gets reset. I cannot stop the line from accumulating a higher spend as the limit can only change next month. I am not happy as it puts me in a very tight space financially.